In the field of vehicles, there is an increase in research and development related to propulsion of vehicles with alternative power sources, i.e. power sources being used as alternatives to conventional internal combustion engines.
An internal combustion engine, for example in the form of a gasoline engine or a diesel engine, offers high efficiency with relatively low fuel consumption. However, environmental concerns have caused an increase in development of more environmental-friendly power sources for vehicles. In particular, the development of electrically operated vehicles has emerged as a promising alternative.
There exist various types of vehicle propulsion systems comprising electric machines. For example, a vehicle can be operated by means of an electric machine solely, or by means of an arrangement comprising both an electric machine and an internal combustion engine. The latter alternative is often referred to as a hybrid vehicle (HEV), and can for example be utilized in a manner in which an internal combustion engine is used for operating the vehicle while driving outside urban areas whereas the electric machine can be used in urban areas or in environments in which there is a need to limit the discharge of harmful pollutants such as carbon monoxide and oxides of nitrogen. According to known technology, electric machines are operated by means of a storage system for electrical energy arranged in the vehicle, typically in the form of a battery unit which is formed by a plurality of rechargeable battery cells and an associated control unit.
In the context of this disclosure, the term “electrically operated vehicles” refers both to pure electric vehicles and to hybrid vehicles.
Furthermore, a vehicle being operated by means of an internal combustion engine and an electric machine supplied with power from a rechargeable electrical energy storage system is often referred to as a plug-in hybrid electric vehicle (PHEV). A plug-in hybrid electric vehicle uses an energy storage system with rechargeable batteries or another suitable energy source which can be restored into a condition involving a full charge through a connection to an external electric power supply.
The technology involved in electrically operated vehicles is closely related to the development of chargeable batteries. Today, lithium-ion batteries are considered as the best battery technology for range, power, and recharging time.
For a driver of an electrically operated vehicle, it is necessary to obtain information related to the capacity of a battery. Such information can be useful for example for determining the remaining distance which can be travelled with the vehicle until the battery is discharged to such a low level that the vehicle cannot be operated. Such a remaining distance is the equivalent in an electrically operated vehicle to a distance which can be travelled with a remaining amount of fuel in a conventional vehicle having a combustion engine which is supplied with diesel or gasoline fuel.
In order to estimate the capacity of a traction battery for a vehicle, it is useful to determine the so-called state of charge (SOC) of the battery. The SOC parameter is normally expressed in percent, and corresponds to the present battery energy capacity as a percentage of its nominal capacity. The SOC can be determined by integrating the battery current over time, based on a start SOC as well as the battery capacity, which varies over time. The battery capacity can be determined by integrating the current over time and linking it to the SOC at start and finish of the integration process.
The SOC parameter is used for example to prevent overcharging of the battery, or to prevent excessive discharging, i.e. it can be used to indicate whether charging of the battery is necessary or not. For example, for a given vehicle it can be recommended to charge the battery when the SOC value has dropped to 20% during driving. Monitoring the SOC parameter is consequently important to prevent excessive discharging of the battery.
There are several ways to determine the SOC parameter. For example, there is a relationship between the SOC and its open circuit voltage, which can be established in the form of a table. This means that, according to known technology, the SOC can be estimated for example by estimating the open circuit voltage of a given battery.
Regarding known technology, it can be noted that the patent document U.S. Pat. No. 5,898,282 teaches a control system for a hybrid vehicle which is configured for calculating the state of charge (SOC) of a battery of the vehicle. The system uses input from sensors measuring vehicle speed, current and voltage levels etc. There is also provided an algorithm which detects the “current operating conditions”, which could be interpreted as referring to “vehicles modes”.
Although there are known methods and arrangements for determining and controlling the SOC of a battery, there are still requirements for more accurately monitoring the energy capacity of a battery, in particular for providing an understanding of how long time the battery can be used for operating a vehicle without being discharged and how long time it would take for fully charging the battery.
Such information can be used for example to determine the remaining distance for the vehicle to travel until the battery needs charging, which is vital information for a driver of such a vehicle.